Tap Changer Condition Assessment Using Dynamic Resistance Measurement

The DVtest method (Dynamic Resistance Measurement - DRM) is an off-line, non-destructive testing method based on a dc current being injected through a winding and a tap changer as it moves through all of its positions. The test current is recorded with high resolution. Also, an on-load tap changer (OLTC) motor current can be recorded simultaneously with the test current. A high sampling rate is very important due to fast transition processes (transition time of a resistor OLTC type is approximately 50 ms). The DRM method has been proven as a very effective method in the early detection of possible faults on on-load tap-changers (OLTCs). This test may be used to detect problems such as slow transition time, open circuits, problems with contacts, transition resistors, mechanism, motor control, and much more. This paper provides the description of the DVtest method for OLTC analysis. In addition, a few interesting cases about detected tap changer defects and diagnostic analysis are presented, as well.